1. Field of the Invention
This invention relates to a holographic information recording/reproducing apparatus for recording information on and reproducing information from a holographic recording medium, utilizing holography.
2. Description of the Related Art
Holographic recording of utilizing holography for recording information on a recording medium is generally realized by laying signal beam that carries image information and reference beam one on the other in the inside of a recording medium and writing the interference fringes thereof on the recording medium. When reproducing the recorded information, the image information is reproduced by irradiating the recording medium with reference beam and causing diffraction to take place by means of the interference fringes.
Volume holography, digital volume holography in particular, has been developed in recent years for practical applications in order to achieve ultra-high density recording. Volume holography is now attracting attention. Volume holography is a technique of actively exploiting the height of a recording medium and three-dimensionally writing interference fringes, and is characterized in that the diffraction efficiency is improved by increasing the height of the recording medium and the recording capacity is raised by multiplex recording.
Digital volume holography is a computer-oriented holographic recording method which uses the same recording medium and recording method as with the volume holography, whereas the image information to be recorded is limited to binary digital patterns.
With digital volume holography, image information that may be analog image information is digitized once and developed into two-dimensional pattern information (page data). Then, the two-dimensional digital pattern information is recorded as image information. When reproducing the recorded information, the digital pattern information is read out and decoded to restore and display the original image.
With such an arrangement, the original information can be reproduced highly reliably by means of differential detection and/or encoding of binarized data for error correction if the SN ratio (signal-to-noise ratio) is relatively poor. Angular multiplexing holographic recording apparatus have been proposed as such means for recording/reproducing information. Angular multiplexing holographic recording apparatus employ a technique of slightly changing the angle of the surface of a holographic recording medium relative to reference beam and signal beam at the time of recording so that different pieces of information can be recorded in a same area in a multiplexed manner. “The InPhase Professional Archive Drive OMA: Design and Function” (Reference Document 1) in 2006 Optical Data Storage Topical Meeting Conference Proceedings MA1 page 3 through 5 describes such a conventional technology.
(Conventional Technology 1)
Now, the holographic information recording/reproducing apparatus described in the Reference Document 1 will be summarily described below for the configuration thereof by referring to FIGS. 6 and 7. This apparatus will be referred to as Conventional Technology 1 hereinafter.
Firstly, the holographic information recording/reproducing apparatus that is adapted to use a transmission-type disk will be described for its recording operation by referring to FIG. 6.
The light beam emitted from a light source (semiconductor laser or the like) 101 with a wavelength of 405 nm is turned into a parallel light beam by a collimator 102. Then, the light beam is transmitted through a HWP (half-wave plate) 103 and split into signal beam 122 and recording reference beam 121 by a PBS (polarization beam splitter) 104. The PBS (polarization beam splitter) 104 is a splitter unit for splitting the light beam from the light source into signal beam and reference beam. The direction of polarization of the light beam is adjusted by rotating the HWP (half-wave plate) 103 to change the balance of intensity between signal beam 122 and recording reference beam 121.
Signal beam 122 that is produced by the split is made to pass through a beam expander 105 so as to be expanded to an optimum size relative to SLM (spatial light modulator) 109. Then, the signal beam 122 is transmitted through a phase mask 106 to produce a uniform intensity distribution on Fourier plane, then reflected by a PBS (polarization beam splitter) 108 by way of a relay lens 107 and then guided to SLM (spatial light modulator) 109.
The direction of polarization of the light beam modulated by the SLM (spatial light modulator) 109 and carrying the given information is rotated by n/2 and the light beam is then transmitted through the PBS (polarization beam splitter) 108. The transmitted light beam is then made to enter object lens 114 by way of a polytopic filter 111, a relay lens 112 and a movable HWP (half-wave plate) 113 and converged onto transmission-type disk 119. Note that the direction of polarization of the light beam is not rotated by the HWP (half-wave plate) 113 for recording.
On the other hand, reflectance light for recording 121 is transmitted through the PBS (polarization beam splitter) 104, reflected by mirrors 115 and 116 and then polarized to a desired direction by a galvano mirror 117. Subsequently, the light beam enters a scanner lens 118 and then passes through the recording position of the transmission-type disk 119 regardless of the direction of polarization by the galvano mirror 117. Thus, angular multiplexing recording is realized by changing the angle of incidence of recording reference beam 121 at the recording position of the transmission-type disk 119 by means of rotation of the galvano mirror 117.
Now, the reproducing operation of the holographic information recording/reproducing apparatus will be described by referring to FIG. 7. The light beam emitted from the light source 101 with a wavelength of 405 nm is turned into a parallel light beam by the collimator 102. Then, the light beam is transmitted through the HWP (half-wave plate) 103 and the PBS (polarization beam splitter) 104 without being reflected. At this time, the direction of polarization of the light beam is adjusted by rotating the HWP (half-wave plate) 103 so that no reflected light may be produced by the PBS (polarization beam splitter) 104.
Then, the light beam is reflected by the mirrors 115, 116 and the galvano mirror 117 before the light beam is transmitted through the transmission-type disk 119 by way of the scanner lens 118 and reflected by a rotary fold mirror 120 so as to follow the same light path. Reproduced light 124 of reference beam for reproduction 123 that enters the transmission-type disk 119 from the opposite direction relative to the recording direction is turned into a parallel light beam by the objective lens 114 and the direction of polarization thereof is changed by n/2 by the HWP (half-wave plate) 113.
Unnecessary reproduced light that is reproduced with desired reproduced light simultaneously has a point of convergence different from that of the desired reproduced light in a plane perpendicular to the optical axis between the lenses of the relay lens 112. Thus, unnecessary reproduced light is eliminated by the polytopic filter 111 that transmits only desired reproduced light. Information is reproduced as reproduced light is reflected further by the PBS (polarization beam splitter) 108 and guided to a CMOS (photodetector) 110.
(Conventional Technology 2)
Now, a holographic information recording/reproducing apparatus adapted to use a reflection-type disk that is disclosed in U.S. patent application Ser. No. 11/864,129 will be described by referring to FIGS. 8, 9, 10A and 10B. This apparatus will be referred to as Conventional Technology 2 hereinafter.
FIG. 8 schematically illustrates the optical system for recording and FIG. 9 schematically illustrates the optical system for reproduction of the Conventional Technology 2. FIGS. 10A and 10B respectively illustrate in detail a light beam striking a reflection-type disk 213 when recording information illustrated in FIG. 8 and a light beam going out from a reflection-type disk 213 when reproducing information illustrated in FIG. 9.
Firstly, the recording operation of the apparatus will be described by referring to FIG. 8. The light beam emitted from a light source (semiconductor laser or the like) 201 with a wavelength of 405 nm is turned into a parallel light beam by a collimator 202 and made to strike a PBS (polarization beam splitter) 203. Then, the light beam is split into signal beam 216 and recording reference beam 215 by the PBS (polarization beam splitter) 203. The PBS (polarization beam splitter) 203 is a splitter unit for splitting the light beam from the light source into signal beam and reference beam. Signal beam 216 produced by the split is transmitted through the PBS (polarization beam splitter) 203 and irradiates an SLM (spatial light modulator) 204.
Then, signal beam 216 is transmitted through the SLM (polarization beam splitter) 204 and then PBS (spatial light modulator) 205 and strikes QWP (quarter-wave plate) 208 by way of a relay lens 207. After being transmitted through the QWP (quarter-wave plate) 208, signal beam 216 is transformed into circular polarized light and made to strike an objective lens 209 so as to be irradiated onto a reflection-type disk 213 as signal beam 216 by the objective lens 209.
Recording reference beam 215 that is reflected by the PBS (polarization beam splitter) 203 strikes a QWP (quarter-wave plate) 211 by way of a relay lens 210. After being transmitted through the QWP (quarter-wave plate) 211, the light beam is transformed into circular polarized light and reflected by a mirror 212 so as to be irradiated onto the reflection-type disk 213 as recording reference beam.
As illustrated in FIG. 10A, the reflection-type disk 213 has a cover layer 220, a recording layer 221, a space layer 222 and a substrate 223, the surface of which forms a reflection plane 224. For recording, signal beam 216 is transmitted through the cover layer 220 in the reflection-type disk 213 and enters the recording layer 221. At this time, recording reference beam 215 is made to enter the reflection-type disk 213 from the upper left as illustrated in FIG. 10A and transmitted through the cover layer 220, the recording layer 221 and the space layer 222 to get to the reflection plane 224.
Assume that the angle of incidence is θ1. Then, the signal beam 216 and the recording reference beam 215 interfere with each other in the recording layer 221 to form interference fringes 218 (219?) and record information. On the other hand, the recording reference beam 215 that is reflected by the reflection plane 224 is transmitted through the space layer 222, the recording layer 221 and the cover layer 220 to leave the reflection-type disk 213. After leaving the reflection-type disk 213, the recording reference beam 215 enters a galvano mirror 214. The angle of the galvano mirror 214 is adjusted such that recording reference beam 215 is controlled so as not to reenter the reflection-type disk 213.
Now, the reproducing operation of the holographic information recording/reproducing apparatus will be described by referring to FIGS. 9 and 10B. Referring to FIG. 9, the light beam emitted from the light source 201 with a wavelength of 405 nm is turned into a parallel light beam by the collimator 202 and made to enter the PBS (polarization beam splitter) 203. Light that enters the PBS (polarization beam splitter) 203 is partly transmitted and partly reflected. Then, the reflected light enters the QWP (quarter-wave plate) 211 by way of the relay lens 210. The light beam transmitted through the QWP (quarter-wave plate) 211 is transformed into circular polarized light and reflected by the mirror 212 so as to be irradiated onto the reflection-type disk 213 as readout reference beam 217.
As illustrated in FIG. 10B, the readout reference beam 217 is made to enter the reflection-type disk 213 from left above and transmitted through the cover layer 220, the recording layer 221 and the space layer 222 to get to the reflection plane 224. Assume that the angle of incidence is θ2. After being reflected by the reflection plane 224, the readout reference beam 217 is transmitted through the space layer 222, the recording layer 221 and the cover layer 220 to leave the reflection-type disk 213. After leaving the reflection-type disk 213, the light is reflected by the galvano mirror 214 and strikes the reflection-type disk 213 once again.
Assume that the angle of incidence of reentering light is θ3. Reproduced light 218 goes out as the readout reference beam 217 is reflected again by the reflection plane 224 and irradiated onto the interference fringes 219 that carry recorded information. The reproduced light 218 that goes out from the reflection-type disk 213 is transmitted through the objective lens 209 and the QWP (quarter-wave plate) 208 to become circular polarized light and enters the PBS (polarization beam splitter) 205 by way of the relay lens 207. The reproduced light 218 that is reflected by the PBS (polarization beam splitter) 205 forms an image on a CMOS (photodetector) 206 to reproduce information.
Note that light transmitted through the PBS (polarization beam splitter) 203 is prevented from being transmitted through the PBS (polarization beam splitter) 205 and getting to the reflection-type disk 213 by way of the objective lens 209 at the time of reproduction. More specifically, a shutter (not illustrated) is arranged between the PBS (polarization beam splitter) 203, the SLM (spatial light modulator) 204 and the PBS (polarization beam splitter) 205 to block the light path and stop light. Anything other than a shutter that can stop light may alternatively be used.
When a transmission-type disk is used with the Conventional Technology 1, the angle of incidence θ1 relative to the transmission-type disk of reference beam reflected by the galvano mirror 117 that operates as first deflector unit at the time of recording is equal to the angle of incidence θ2 relative to the transmission-type disk of reference beam reflected by the first deflector unit at the time of reproduction.
Similarly, the angle of incidence θ3 relative to the transmission-type disk of reference beam coming out from the transmission-type disk and made to reenter the transmission-type disk by the galvano mirror 120 that operates as second deflector unit is equal to the angle of incidence θ1.
Thus, the optical axis of reference beam entering the transmission-type disk and the optical axis of reference beam reentering the transmission-type disk agree with each other at the time of reproduction to give rise to stray light and aggravate the SN (signal to noise) ratio when reference beam is transmitted through the transmission-type disk.
When, on the other hand, a reflection-type disk is used with the Conventional Technology 2, the angle of incidence θ1 relative to the reflection-type disk of reference beam reflected by the mirror 212 that operates as first deflector unit at the time of recording is equal to the angle of incidence θ2 relative to the reflection-type disk of reference beam reflected by the first deflector unit at the time of reproduction. (See FIGS. 10A and 10B.)
Similarly, the angle of incidence θ3 relative to the reflection-type disk of reference beam coming out from the reflection-type disk and made to reenter the reflection-type disk by the galvano mirror 214 that operates as second deflector unit is equal to the angle of incidence θ1. (See FIGS. 10A and 10B.)
Thus, there arises a problem that the angle of incidence θ2 of reference beam entering the reflection-type disk at the time of reproduction is equal to the angle of incidence θ3 of reference beam reentering the reflection-type disk to give rise to stray light and aggravate the SN (signal to noise) ratio.